Some thin films useful in the manufacture of semiconductor devices may include significant amounts of hydrogen either by choice or by the inherent result of the method of film formation. For example, one may choose to include hydrogen in a silicon film to reduce carrier trapping at dangling bonds or grain boundaries, effective to provide a higher carrier mobility. Further, plasma enhanced chemical vapor deposition from silane (SiH.sub.4) inherently produces silicon and silicon nitride films that include a significant amount of hydrogen. This is particularly true if the deposition is performed below about 400.degree. C. In this latter connection, it should be recognized that plasma enhanced chemical vapor deposition is a very useful process for forming films on substrates that cannot be subjected to extremely high temperatures, as for example glass. One application might be a multiplexed liquid crystal display module in which a glass plate forming part of the liquid crystal display also supports one or more islands of silicon in which semiconductive devices controlling the display are formed.
When forming semiconductive devices in a plasma enhanced chemical vapor deposited silicon layer formed at 300.degree.-400.degree. C., a significant proportion, i.e. about 10-30 atomic percent, of hydrogen will be included in the coating. It is to be noted that the precise proportion of hydrogen in the layer is one of the factors that determines the precise characterictics of the resultant semiconductive devices formed in that layer. On the other hand, it is extremely difficult to control deposition processes such as plasma enhanced chemical vapor deposition accurately enough to obtain a precisely predetermined hydrogen content in the resultant film. Where such a precise predetermined hydrogen content is desired, it can be obtained by producing more hydrogen in the film than is eventually desired, measuring the actual resultant hydrogen content, and then annealing, i.e. heat treating, the film to out-diffuse the hydrogen down to the desired predetermined level. Annealing can also be used merely to reduce hydrogen content to some low level significantly below the deposition level, if the deposition level is inherently too high for some reason. In fact, annealing can be used to remove substantially all the hydrogen, when no appreciable hydrogen content is desired.
The use of annealing to out-diffuse an undesired component is, of course, not new. On the other hand, when thin films containing significant proportions of hydrogen are heat treated, even at relatively moderate temperatures, hydrogen evolution occurs disruptively. Hydrogen gathers at localized sites in and under the film, forming bubbles that blister the film. Blistering is so extensive in films containing more than about 15 to 20 atomic percent hydrogen that films are no longer useful for most intended applications. For example, a satisfactory semiconductive device cannot be formed in a blistered area of film. If the heating is more rapid and/or the temperatures higher, an even lower amount of atomic hydrogen, as for example about 10 atomic percent, can produce damaging blistering also.
It should also be recognized that in some cases, the hydrogen inherently included in the film due to the deposition may be a problem primarily because the film is inherently subjected to a heating step later on. If disruptive hydrogen evolution occurs, and if the film is a passivating or sealing coating, its protection is at least deleteriously affected, if not entirely lost.
I have found that if such films are given a significant ion implantation before they are heated, disruptive hydrogen evolution, that is, blistering, does not occur. Accordingly, hydrogen content of hydrogen-containing films can be precisely adjusted down to any desired level by annealing without damaging them. Why this occurs is not understood. On the other hand, experimental evidence indicates that ions implanted in a sufficient dosage and depth may serve to activate atoms in the film. This may enhance atomic or molecular out-diffusion of hydrogen from the film, so that it occurs more uniformly across the film surface. In any event, hydrogen does not accumulate in bubbles that blister the resultant film.